Two light level lighting systems have been utilized in overhead lighting for many years. Typically, two light level systems are implemented by using two power switches and two ballasts in each lighting fixture, wherein each of the power switches controls only one of the ballasts in the fixture. Turning on both of the switches at the same time powers both ballasts, thus producing full light output from the fixture. Turning on only one of the switches applies power to only one of the ballasts in the lighting fixture and results in a reduced light level and a corresponding reduction in power consumed.
Because it is more economical to have a single ballast in the fixture instead of two, a system for producing the same result using only a single ballast is desirable. For compatibility purposes, the ballast would be required to operate from the same two power switches used in the two ballast system. When both switches are closed, the ballast would operate in a full light mode. Conversely, when only one of the two power switches is closed, the ballast would operate in a reduced light mode.
Two light level systems that require only a single ballast are known in the art. For example, U.S. Pat. No. 5,831,395 (issued to Mortimer) discloses one such system, which is described in FIG. 1. As shown in FIG. 1, the Mortimer system includes a detector circuit 270 that provides a control signal that is dependent on the states of two on-off switches S1 and S2. Theoretically, when only one of the switches S1,S2 is on, the control signal will be at a first level, causing the ballast to drive the lamp at a reduced light level; when both of the switches S1,S2 are on, the control signal will be at a second level, causing the ballast to drive the lamp at a higher light level.
Unfortunately, the Mortimer system has a major limitation in that detector circuit 270 may not function properly in the presence of X capacitances that are typically present between the hot and neutral wires that connect the ballast to the switches S1,S2 and the AC source. These X capacitances (denoted by dashed line/phantom capacitor symbols in FIG. 1) are present due to EMI circuitry in the ballast and/or the nature and length of the wiring between the AC source, switches S1,S2, and the ballast. Essentially, these X capacitances compromise the ability of detector circuit 270 to distinguish between a condition where only one switch is closed versus a condition where both switches are closed, and thus defeat the intended functionality of a two light level approach. This problem is particularly pronounced when multiple ballasts are connected to the same branch circuit, in which case the X capacitances due to the EMI circuitry in each ballast, and/or the wiring between the AC source, switches S1,S2, and each ballast, are additive.
What is needed, therefore, is a ballast that provides two light levels but that is substantially insensitive to the capacitances that are typically present in actual lighting installations. Such a ballast would represent a significant advance over the prior art.